The knee joint is one of the strongest joints in the body because of the powerful ligaments that bind the femur and tibia together. Although the structure of the knee provides one of the strongest joints of the body, the knee may be one of the most frequently injured joints, e.g., athletes frequently stress and tear knee ligaments. The large number of ligament injuries has given rise to considerable innovative surgical procedures and devices for replacing and reconstructing torn or dislocated ligaments, typically involving grafting autografts, allografts, or a synthetic construct, to the site of a torn or dislocated ligament. For example, the replacement of an anterior cruciate ligament (ACL) may involve transplanting a portion of the patellar tendon, looped together portions of semitendinosus-gracilis (hamstring) tendons, or donor Achilles tendons, to attachment sites in the region of the knee joint.
Tears or ruptures of an anterior cruciate ligament of a knee (ACL) typically require a major surgical intervention wherein a replacement graft is mounted to the ends of the bones surrounding the knee in order to reconstruct the knee. A ruptured or damaged ACL typically results in serious symptoms such as knee instability resulting in diminished ability to perform high level or recreational sports, or in some cases daily activities relating to motility. Although the use of knee braces may alleviate some of these symptoms, the potential long term effects of a damaged ACL include meniscal damage and articular cartilage damage.
The basic steps in a conventional ACL reconstruction procedure include: harvesting a graft made from a portion of the patellar tendon with attached bone blocks; preparing the graft attachment site (e.g., drilling holes in opposing bones of the joint in which the graft will be placed); placing the graft in the graft attachment site; and rigidly fixing the bone blocks in place within the graft site, i.e., the holes or “bone tunnels”. The screws used to fix the graft in place are called “interference screws” because they are wedged between the bone block and the wall of the bone tunnel into which the bone block fits. Typically, there is very little space between the bone block and the inner wall of the bone tunnel in the bone at the fixation site.
Several types of surgical procedures have been developed to replace the ACL. Although repair would be a preferred procedure, it is not typically possible since the end of the torn ACL is typically not of sufficient length to reattach successfully. However, reconstructions can be made to a damaged ACL.
There are several types of conventional replacement grafts that may be used in these replacement procedures. In all procedures tibial and femoral tunnels are drilled by the surgeon using conventional techniques. Known, conventional drill guides and drills are used. In one type of procedure known as a bone-tendon-bone procedure, an autograft tendon is harvested from the patellar tendon along with an attached bone block on one end harvested from the patella and a harvested bone block on the other end harvested from the tibia. In order to secure the graft in the knee, one end is mounted into the tibial tunnel and other end is mounted into the femoral tunnel. This is done by mounting the opposed bone blocks in the tibial and femoral tunnels, respectively, in the following manner. A guide pin is passed through the tibial tunnel, into the fermoral tunnel and out through the lateral femoral cortex. Suture is used to attach the graft to the proximal end of the guide pin. The distal end of the guide pin is then pulled out of the lateral cortex of the femur and the graft is pulled into the knee (femoral and tibial tunnels). Once the bone blocks are emplaced in the respective tibial and femoral tunnels, the blocks are secured in place in the following manner. One method of securing or fixing the ends of the graft in the tunnels is to use a conventional metallic interference screw. The screw is inserted into the opening of a tunnel and placed in between the graft and the interior surface of the bone tunnel. It is then turned and screwed into the tunnels, thereby forcing the end of the graft against an interior surface of the bone tunnel. The ends of graft are secured and maintained in place in the tunnel by means of a force fit provided by the interference screw.
Another surgical procedure for the replacement of an anterior cruciate ligament involves providing a graft ligament without attached bone blocks. The graft can be an autograft or an allograft. The autografts that are used may typically be harvested from the hamstring tendons or the quadriceps tendons. The allografts that are conventionally used are harvested from cadaveric sources, and may include the hamstring tendons, quadriceps tendons Achilles tendon, and tibialus tendons. If desired, and if readily available, it may possible to use synthetic grafts or xenografts. Tibial and femoral tunnels are similarly drilled in the tibia and femur respectively using conventional techniques, drill guides and drills. Once the tunnels have been drilled, the surgeon then pulls the graft through the tibial and femoral tunnels using conventional techniques such that one end of the graft resides in the tibial tunnel and the other end of the graft resides in the femoral tunnel. For example, one conventional technique for pulling a graft through the tunnels is to attach the graft to the proximal end of a guide pin using conventional surgical suture. The guide pin is then passed through the tibial tunnel, into the femoral tunnel, and out though the femoral cortex. The distal end of the guide pin is then pulled out of the lateral cortex of the femur and the graft is pulled into the knee (femoral and tibial tunnels). After the surgeon has emplaced and positioned the ends of the graft in the respective tunnels, the graft ends need to be secured and fixed in place to complete the replacement procedure. One method of securing or fixing the ends of the graft in the tunnels is to use a conventional metallic interference screw. The screw is inserted into the opening of a tunnel and placed in between the graft and the interior surface of the bone tunnel. It is then turned and screwed into the tunnels, thereby forcing the end of the graft against an interior surface of the bone tunnel. The ends of the graft are secured and maintained in place in the tunnel by means of a force fit provided by the bone screw.
Interference screws for anchoring ligaments to bone are typically fabricated from medically approved metallic materials that are not naturally degraded by the body. One potential disadvantage of such screws is that once healing is complete, the screw remains in the bone. An additional disadvantage of a metal screw is that in the event of a subsequent rupture or tear of the graft, it may be necessary to remove the metal screw from the bone site. Metallic screws may include a threaded shank joined to an enlarged head having a transverse slot or hexagonal socket formed therein to engage, respectively, a similarly configured, single blade or hexagonal rotatable driver for turning the screw into the bone. The enlarged heads on such screws can protrude from the bone tunnel and can cause chronic irritation and inflammation of surrounding body tissue.
Permanent metallic medical screws in movable joints can, in certain instances, cause abrading of ligaments during normal motion of the joint. Screws occasionally back out after insertion, protruding into surrounding tissue and causing discomfort. Furthermore, permanent metallic screws and fixation devices may shield the bone from beneficial stresses after healing. It has been shown that moderate periodic stress on bone tissue, such as the stress produced by exercise, helps to prevent decalcification of the bone. Under some conditions, the stress shielding which results from the long term use of metal bone fixation devices can lead to osteoporosis.
Biodegradable interference screws have been proposed to avoid the necessity of surgical removal after healing. Because the degradation of a biodegradable screw occurs over a period of time, support load is transferred gradually to the bone as it heals. This reduces potential stress shielding effects.
In order to overcome the disadvantages that may be associated with metal interference screws, interference screws made from biodegradable polymers are known in this art. For example, it is known to use an interference screw made from polylactic acid. Ideally, the biodegradable interference screw will rapidly absorb or break down and be replaced by bone. However, it is known that screws made from polylactic acid tend to maintain their structural integrity for very long periods of time thereby preventing the desired bone in growth. Attempts have been made to improve the bone regeneration process by using other biodegradable polymers and copolymers of lactic acid that resorb or absorb more quickly. The problem often associated with these quicker absorbing polymers or copolymers is that the bone regeneration may proceed at a much slower rate than the rate of resorption, resulting in premature mechanical failure of the screw and a resulting pull out of the graft end from the femoral tunnel. Some of the absorbable interference screws of the prior art may take several years to absorb, and may result in a fibrous tissue mass or cyst being left behind, not bone. This lack of bone in-growth may create fixation problems if the ACL is torn again, necessitating a new graft replacement. In addition, if the screw absorbs too slowly, the screw will need to be removed in the event of a subsequent failure of the graft.
Accordingly, what is needed in this art is a novel method of performing an ACL replacement graft procedure using a novel interference screw made from a biodegradable material which rapidly absorbs or degrades and promotes bone in-growth.